Hello Scott, welcome to the forums.
Not mentioned yet is the series of books "British Single Shot Rifles" by Wal Winfer.
Volume 4 deals with Westley Richards & Co. and has quite a section devoted to the Model 1881. Photos, drawings, and descriptions should help you decide what "look" you want on a restored rifle.
The last part of the book features many of the Westley Richards and other cartridges that were used in the rifles, .303 on up to .500BPE.
Worth the price of the book for anyone restoring one.

The .500/.450 #1 Express is a fine cartridge, I have a Cogswell & Harrison in that caliber, and it is quite an accurate rifle. The caliber is a totally handload proposition though; the base and rim are not common to more available cartridge brass but .450 #2 Nitro brass or even .50 Sharps 3 1/4 can be formed to work. The problem is rim thickness if chambered to original spec. The WR .500/.450 #1 Express, unlike other .500 Brit based cartridges, has a very thick .060" rim, the common .500 express/Nitro express brass with .040" thick rim will not work unkess you headspace on the case shoulder or build up the rim thickness.

I hope the "gunsmith" that hosed that rifle up with that conversion you described did not screw the extractor up too badly.
- Mike